103's would have gone in a gunpod, similar to the 4x 20mm option.
The 110 could have done a lot more in terms of ground attack than the mossie, had research been put into it. But it looks like they just shoved it aside for the most part after 1942, which is not the case with the mossie.
Sure the mossie would have been a better fighter and bomber (internal bomb bay), but not THAT much better as a fighter. Even now the 110G isn't a terrible match for the mossie.
Like I said your bias wont let you see the forest for the trees!
First check the Cd on the Mossie,then compare it to the 110,then calculate how much HP the 110 would need to match,say about 425 mph,I'm positive the mossie could go faster but 425 is a nice round number.
Once you've done that,find and engine in the Luft inventory that could produce the required HP.
There is no way the 110 can compete,not speed,ords or fighting ability. In AH I used to fly the mossie a fair bit,several hundred hours,lately I've been fly German planes. I fly the 110C a fair amount,the 110C can turn with the mossie but the mossie can dictate the fight with superior speed and climb.
Oh and adding 103's would just overload an already overloaded airframe! Also the Mossie can counter that with the 6 pounder,oh and it actually carried that gun unlike the 110 and the whatif 103's.
